A radial turbine, when it is used as the exhaust turbine of a turbocharger as often is the case, can accomplish a high degree of supercharging even when the speed of the exhaust gas entering the turbine is low by reducing the size of the nozzles defined adjacent to the periphery of the turbine wheel to a small value and thereby increasing the speed of the exhaust gas flow directed to the turbine wheel. On the other hand, in high speed range, narrowing the nozzles causes the efficiency of the engine to drop because the resistance to the flow of the exhaust gas increases and a considerable back pressure is created in the exhaust system of the engine.
Such a property of the radial turbine for a turbocharger is characterized by the ratio of the cross-sectional area A of the throat section of the scroll passage to the distance R between the center of the cross-section and the center of the turbine wheel. When this ratio A/R is small, the speed of the exhaust gas directed to the turbine wheel is accelerated and a high degree of supercharging is possible even in low speed range, but a significant back pressure is produced in the exhaust system in high speed range. On the other hand, when this ratio A/R is large, the turbine produces a relatively low back pressure even in high speed range but the speed of the exhaust gas directed to the turbine wheel is relatively so low in low speed range that a sufficient degree of supercharging is possible only in a relatively high speed range.
According to U.S. Pat. No. 3,101 926 issued to Weber and U.S. Pat. No. 2,860,827 issued to Egli, this problem is avoided by rotating, around axial pivot pins, a plurality of moveable vanes arranged around the periphery of the turbine wheel to vary the opening area of the nozzles defined between the adjacent vanes. According to these proposals, a sufficient supercharging effect is obtained even in low speed range of the engine by narrowing the nozzles, and the back pressure working against the exhaust gas of the engine is reduced in medium to high speed range by increasing the size of the nozzles.
However, according to these prior inventions, since the moveable vanes are arranged in such a region where the speed of the fluid is relatively high, the resistance loss of the fluid flow is accordingly high, and, therefore, not only the efficiency of the turbine is reduced but also, because the opening area of the nozzles between adjacent moveable vanes changes considerably even for a small change in the angle of the moveable vanes particularly when the opening area is small, desirable precision in control is not easy to obtain.
Further, it is also known to define a part of the wall of the scroll passage with a flap which is capable of a swinging motion to vary the A/R ratio, for instance, from U.S. Pat. No. 4,678,397 issued to Komatsu, for instance, but its range of nozzle area variation is not necessarily wide enough, and, further, particularly when the flap opening angle is large, the fluid flow directed towards the turbine wheel becomes so disturbed and uneven that the turbine efficiency drops.
To eliminate such problems, an improved variable capacity turbine was proposed in copending U.S. patent application Ser. No. 054,499, filed May 27, 1987, which comprises a plurality of arcuate fixed vanes arranged around a throat section defined around the periphery of a turbine wheel, and moveable vanes which vary the nozzle area defined between the moveable vanes and the fixed vanes. However, according to this proposal, a certain difficulty was encountered in further expanding the range of the A/R ratio control because the moveable vanes were moved at a fixed control precision irrespective of the angle of the moveable vanes, and a fine control of the nozzle opening area was not possible for a given range of exhaust gas flow rate. If the control system is tuned for a fine adjustment of the nozzle opening area in low nozzle opening range, the turbine will be incapable of handling a large flow rate of the exhaust gas without causing a significant increase in the back pressure in the exhaust system.